Yes, you can add as an Admin a coupon your customer subscription.
Option A: Add Coupon on a new subscription created through the App
Step1: Go to Customers > Subscription (tab) > New Subscription
Step 2: Add Coupon Code while you are creating a new subscription to the customer
Option B: Add Coupon on existing subscription
Step1: Go to Customers > Coupon (tab)
Step 2: Add Coupon Code > Select Subscription > Select Coupon